Chernobyl HBO Chernobyl physics questions

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Some questions regarding the physics that lead to the disaster. Mostly influenced by the HBO series.
Just finished that amazing HBO series "Chernobyl". As a physicist I am very much impressed by the explanation given by Legasov in the last episode. Especially the part he clarifies how the reactor was in an unstable equilibrium minutes before the event with nearly all the rods pulled out but choked by Xenon at the same time.

Most of the stuff maked sense. (It is so detailed though that I would not be able to understand from wikipedia on its own without simplified and nicely categorized explanation in the last episode.) But still some questions remain.

1. He says water reduces reactivity but steam increases it. How does this really work?
2. He says heat reduces reactivity. (Completely hypothesizing here, is it something to do with phonons interacting with neutrons? I might be out of context though, if so please correct.)
3. The deadly flaw about the rods: their graphite tips. He says graphite increases reactivity. But isn't it used for "moderating" purposes in the reactors all over the globe? An elaborate explanation I would appreciate.
 

anorlunda

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Caveat: I am not a nuclear engineer. But I have some acquaintance with the technical issues.

The answers are not simple.

Boiling (voids) reduce the density of water, and thus reduces the slowing down of neutrons. But it also shifts the spectrum of thermal energies. (Not every neutron has the same energy, we have a spectrum of energies.) Because of the extreme nonlinearities in the U235 fission cross section versus energy, the effects of an incremental change can have positive or negative components.
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There are similar nonlinear curves for all the possible reactions with all the fission products present.

Overall, net reactivity change is the sum of numerous positive and negative terms and effects, most of which are nonlinear, and also that may have substantially different values in different operating regimes. That makes calculating the net tedious to say the least.

It also makes it impossible to give definitive simple answers to questions like yours. "It depends," is hardly a satisfying answer, but short of answering a sentence with a curve, it is the most accurate thing to say.

Reactor designs with positive void coefficients are supposed to be rejected because it is so destabilizing. The impression I got from the TV show, was that the RBMK positive void coefficient slipped through the cracks because it was only positive in a peculiar regime where the reactor was not expected to ever operate, but they got there during that bungled test.
 

member659127

That is a beautiful plot, thanks! I agree that it is nothing trivial and respected nuclear scientists and engineers even more after seeing the show. :smile: One other thing that catched me is even a "simplified" explanation for the general audience is not easy to grasp.

Reactor designs with positive void coefficients are supposed to be rejected because it is so destabilizing. The impression I got from the TV show, was that the RBMK positive void coefficient slipped through the cracks because it was only positive in a peculiar regime where the reactor was not expected to ever operate, but they got there during that bungled test.
That also makes sense. Still curious about the graphite thing though.
 

Orodruin

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Summary: Some questions regarding the physics that lead to the disaster. Mostly influenced by the HBO series.

The deadly flaw about the rods: their graphite tips. He says graphite increases reactivity. But isn't it used for "moderating" purposes in the reactors all over the globe? An elaborate explanation I would appreciate.
Moderation does not mean what you think it means. The neutrons resulting from the fission are too fast to cause the chain reaction you are looking for. Hence, you need to slow them down to speeds where the fission cross section is sufficiently large. This is the purpose of the moderator.
 

member659127

Moderation does not mean what you think it means. The neutrons resulting from the fission are too fast to cause the chain reaction you are looking for. Hence, you need to slow them down to speeds where the fission cross section is sufficiently large. This is the purpose of the moderator.
So you are saying that by inserting graphite between the high energy neutrons I "moderate" them to create even more reactions?
 

anorlunda

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So you are saying that by inserting graphite between the high energy neutrons I "moderate" them to create even more reactions?
Yes. Both the graphite and the water moderate (slow down) the neutrons. so called thermal reactors need the higher probability of fission at low energies.

You can see that on the plot in #2. The neutrons begin at the energy of the bjuds vertical region of the plot. They must be slowed down before triggering more fission events.
 

member659127

You can see that on the plot in #2. The neutrons begin at the energy of the bjuds vertical region of the plot. They must be slowed down before triggering more fission events.
Ok, then first just to understand that correctly. The blue region marked as "fission neutron energy" is the regime where the neutrons are created as a result of fission? And I want them to be slowed down to the "resonance region" to increase reaction rate?

Now, if that is true that brings another question to my mind. If someone wants to create an uncontrolled reaction deliberately like in the case of a nuclear bomb; than this mechanism will work "against" the chain reaction? How is it managed in that case then?

Very informative answers, thanks to both by the way.
 

anorlunda

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Ok, then first just to understand that correctly. The blue region marked as "fission neutron energy" is the regime where the neutrons are created as a result of fission? And I want them to be slowed down to the "resonance region" to increase reaction rate?

Now, if that is true that brings another question to my mind. If someone wants to create an uncontrolled reaction deliberately like in the case of a nuclear bomb; than this mechanism will work "against" the chain reaction? How is it managed in that case then?
You are catching on fast.

A bit of nomenclature. We were discussing "thermal rectors" where the word thermal means that the neutrons are thermalized before causing new fissions. Most power reactors are of the thermal type.

There are also "fast reactors" with controlled reactions using so-called prompt neutrons that have not been slowed. In the plot from #2, there are labels on the horizontal axis saying "thermal" and "fast".

A nuclear explosion is an uncontrolled reaction using by-definition prompt neutrons.

A more general understanding comes from simply contemplating chain reactions. What does it take to make a chain reaction exactly critical, with a constant number of events per second?
 

rpp

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You are asking good questions...

> 1. He says water reduces reactivity but steam increases it. How does this really work?

You're first sentence is really saying the same thing. In the case of the Chernobyl reactors, water acts as a neutron absorber. if you add more water, you absorb more neutrons and reactivity decreases. If the water boils, you have less water, so reactivity increases. Why this matters is that when the power increases, the reactivity increases, and this is a really bad design feature (called a positive reactivity coefficient).

Another thing to remember is that neutron moderation is performed with graphite blocks, which are stationary at all power levels.

Contrast this to light water reactors (LWR's) where the water is both the coolant and the moderator. When the power increases, the water density decreases, which decreases the moderation as well as the coolant. Therefore, LWR's have a negative power coefficient. When the power increases, the reactivity decreases.



> 2. He says heat reduces reactivity. (Completely hypothesizing here, is it something to do with phonons interacting with neutrons? I might be out of context though, if so please correct.)

This is something called the "Doppler effect". In the figure in comment:2, you can see "spikes" in the cross sections. These spikes are called resonances. When the temperature increases, the resonances increase in width, and absorption increases. This causes a decrease in reactivity.

So when the power goes up, the temperature goes up, and the reactivity decreases due to Doppler. This is also a negative reactivity coefficient and another very important safety feature.

The Doppler effect occurs in every reactor that has uranium or plutonium fuel (i.e. every reactor)


> 3. The deadly flaw about the rods: their graphite tips. He says graphite increases reactivity. But isn't it used for "moderating" purposes in the reactors all over the globe? An elaborate explanation I would appreciate.

Yes, you are correct. Graphite is a moderator. The key here is that the graphite is located on the tip of the control rod. Therefore, when you insert a control rod, you are inserting graphite and increasing reactivity. This is a bad thing, when you insert a control rod you expect that the reactivity will go down, not go up.

The graphite tips are complicated because the total reactivity depends on where the graphite tip is, and where the absorber regions are located relative the core. Almost always, the control rod is located where the neutron absorber has a much bigger affect than the tip, so the control rod is always negative reactivity. However, in the accident, they had the control rods withdrawn too much and the tip was located at the very top of the core and the absorber was completely withdrawn. Therefore, when the initially inserted the control rod back in the core, the tip was dominate and positive reactivity was inserted. This is what initiated the accident, but it was due to the reactor being in a very unsafe initial condition. The operators had to do a lot of things wrong to get the reactor into this condition.
 

member659127

Thanks for the nice explanation especially for the first question. Few more questions if you dont mind as I have found a nuclear engineer PhD. :smile:

This is something called the "Doppler effect".
"Doppler broadening" phenomenon as commonly known right? If so I get it.
But what are the "resonances" in the first place. Resonance of what?

The key here is that the graphite is located on the tip of the control rod.
They can not be plain stupid right? I feel like there should be a rationale here using it in the "tip" beside "being cheap" as quoted in the final episode. I mean using a moderator in the tip of an absorber. Just doesnt make sense.
 

anorlunda

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"Doppler broadening" phenomenon as commonly known right? If so I get it.
But what are the "resonances" in the first place. Resonance of what?
They are those peaks in the plot in #2. It's just semantics. Each peak resembles a resonance peak from an electrical circuit. They might have been called spires instead, making it sound like there was a connection to church steeples.

Edit: the peaks may be caused by oscillation modes of nucleons within a nucleus. That is deep physics, where nuclear engineers seldom need to go.


They can not be plain stupid right? I feel like there should be a rationale here using it in the "tip" beside "being cheap" as quoted in the final episode. I mean using a moderator in the tip of an absorber. Just doesnt make sense.
In addition to absorb/moderate, every material has properties of strength, weight, ductility, immunity to radiation embrittlement, and so on. Zirconium is a common solution. Nevertheless, you can't make a statement like that without examining the design choice holistically.
 

member659127

Edit: the peaks may be caused by oscillation modes of nucleons within a nucleus. That is deep physics, where nuclear engineers seldom need to go.
Yes, that "deep" part I was concerned and it seems it is related to what you are saying.

Anyway doing some reading on wikipedia, it says:

"Although the shape of resonances changes with temperature, the total area under the resonance remains essentially constant. But this does not imply constant neutron absorption. Despite the constant area under resonance a resonance integral, which determines the absorption, increases with increasing target temperature. This, of course, decreases coefficient k (negative reactivity is inserted). "

That is a very confusing statement. Constant area = constant integral but it says integral increases. And even in that case how an increase in absorption (by target nuclei) would decrease reactivity ???

In addition to absorb/moderate, every material has properties of strength, weight, ductility, immunity to radiation embrittlement, and so on. Zirconium is a common solution. Nevertheless, you can't make a statement like that without examining the design choice holistically.
Let me rephrase then, why do you need a "tip" in the first place? Just do it from the same material all the way down and up.
 

anorlunda

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That is a very confusing statement. Constant area = constant integral but it says integral increases. And even in that case how an increase in absorption (by target nuclei) would decrease reactivity ???
That curve in #2 is a cross-section. It depicts the probability of an event. To get the number of events you need to integrate the probability times the density of neutrons with respect to energy. Change the temperature (or the void fraction), and the energy distribution of the neutrons changes too.

Let me rephrase then, why do you need a "tip" in the first place? Just do it from the same material all the way down and up.
The boron might crumble and fall away in pieces. As I said before, materials have many important non-nuclear properties.
 

member659127

Change the temperature (or the void fraction), and the energy distribution of the neutrons changes too.
which leads to what overall? Increase in absorption by target nuclei? If so how can reactivity decrease with increasing absorption?
 

anorlunda

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which leads to what overall? I
Changes in reactivity. That's what we've been talking about all along isn't it?

By the way, that curve in #2 is for fission events. There are also curves for absorption events and scattering events. Also some neutrons escape and some decay.

The process is complex and it does not lend itself to such simple questions as yours. Each answer is likely to lead to another question. If your interest is enough for some study, I suggest starting here, then following with the references cited in the Wiki. Just asking questions on the Internet is not a good learning strategy.
https://en.wikipedia.org/wiki/Nuclear_chain_reaction
 

mathman

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Has any one made the point about the control rod problem? They got stuck before they could be inserted, so only the graphite tips were in and the boron wasn't working.
 

anorlunda

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Has any one made the point about the control rod problem? They got stuck before they could be inserted, so only the graphite tips were in and the boron wasn't working.
Edit: Yes, we have been discussing the control rods. The failure to insert is another factor/matter of concern with respect to the accident.
 
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Astronuc

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Let me rephrase then, why do you need a "tip" in the first place? Just do it from the same material all the way down and up.
The idea of the graphite tip was to compensate for the build-up of Xe-135m/Xe-135 as the neutron flux decreased, which is not a good idea if the spectrum hardens, which reduces the negative reactivity associated with the Xe-buildup, and it is especially inappropriate if the tips are inserted during a reactor positive reactivity transient, since the graphite types add positive reactivity before the neutron poison (absorbing) sections introduce negative reactivity. The fact that the control rods didn't fully insert indicates core damage before the control rods were inserted, i.e., they were effected too late during the transient.

I'm finding different numbers for the minimum number of control rods required to be in the RBMK core, 26, 28 or 30, but apparently there were much less, ~18.

From one text on reactor physics:
Reactivity control in RBMK reactors is largely associated with the control rod infrastructure. This is organised into four groups comprising: a set of shortened absorber rods for axial control (× 24, length ~ 3 m), full-length (~ 5 m) absorbing rods for radial control (× 24), absorbing rods for the automatic control of reactor power (× 139) and emergency rods (× 24). There are 211 in total and all are made of boron carbide. RBMK control rods are distinguished in one way in particular by the graphite displacers (or ‘followers’) that form part of their design. These are designed to remain in the core when the absorbers are withdrawn and act to displace the light-water coolant from the space that would otherwise be left behind. Without these, undesirable neutron absorption on the hydrogen in the water that would fill the space left by the rod would occur and undermine neutron economy.
An important piece in the quote is the sentence, "These (the graphite followers) are designed to remain in the core when the absorbers are withdrawn and act to displace the light-water coolant from the space that would otherwise be left behind."
Ref: https://www.sciencedirect.com/topics/engineering/reactivity-control, see the Section 10.8.4 Reactivity Control and Protection from Malcolm Joyce's Nuclear Engineering, 2018.

I also read that the time to insert control rods was 18 seconds, which was subsequently reduced to 12 seconds in safety upgrades!

Seriously?! PWR scram time is ~2 seconds, roughly the time for the reactor protection system to activate and de-energize the magnets suspending the control rods above the core, then have control rods free-fall ~12 feet into the core (3.6).
 
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member659127

I have been teaching physics longer than I have studied it. So I know the difference when somebody talks about something he doesn't have confidence and somebody simply knows it.

The idea of the graphite tip was to compensate for the build-up of Xe-135m/Xe-135 as the neutron flux decreased, which is not a good idea if the spectrum hardens, which reduces the negative reactivity associated with the Xe-buildup, and it is especially inappropriate if the tips are inserted during a reactor positive reactivity transient, since the graphite types add positive reactivity before the neutron poison (absorbing) sections introduce negative reactivity.
That's what I am talking about. Thank you sir!
 
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And they probably couldn't simply keep the moderator channels empty because they are part of the core and so would overheat and be damaged so they too had light water in those channels which then as @Astronuc already said caused extra neutron absorption so they eventually settled with the graphite tip.

@rpp already said this quite nicely, but let me add, the problem with positive void coefficient in the RBMK is so dramatic because the core is always surrounded by graphite and graphite just moderates neutrons (slows them down and reflects them) but is very weak at absorbing them, light water on the other hand absorbs neutrons more, much more. So as the water turns into steam in the core , the core loses neutron absorption but does not loose neutron moderation.
In Chernobyl the core was in a position where neutrons were absorbed by xenon and light water coolant passing through (as most control rods were out of the core), in this choked state Dyatlov ordered to start the test anyway and so they had to switch off the turbine, they stopped steam flow to the turbine and sharply decreased water flow through the core , the water in the pressure channels started to boil away so neutron absorption decreased but graphite was still there so the core still had the full potential of moderating neutrons, adding to this the fuel itself was colder than it would be if the core was operating at it's designed thermal output,

https://energyeducation.ca/encyclopedia/Neutron_moderator
see the link for neutron absorption etc properties


as was pointed out earlier if the fuel is colder it's reactivity is higher and when it becomes hotter it;'s reactivity decreases due to doppler effect.
So the fuel was ready to react, water was evaporating and the core was loosing neutron moderation capability, at some point when this happened the core managed to burn away the xenon enough apparently and so the exponential increase in reactivity began. This increase is much faster than mechanical processes like the movement of control rods, so it probably was already too late even if the rods did not have the graphite on them, but just to add insult to injury the rods had graphite on them which probably doubled down on the whole runaway situation.
 

Astronuc

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I'm still puzzled by the control element (rod) design(s). In Malcolm Joyce's summary, there are 24 emergency control rods, which I suspect do not have graphite tips, but I cannot find detailed drawing/specs. I understand that power regulation rods would have graphite tips, which may be fine for normal operation, but a terrible concept with respect to fast transients, such at that initiated during the experiment leading to the destruction of the core of Chernobyl 4. I hesitate to call it an accident, since it was deliberate and negligent.

No such experiment would be allowed in commercial power reactors, at least not with the reactors and utilities with which I've been involved. Any kind of test must be fully evaluated, well before implementation. I'm aware of special test configurations on individual rods and small test reactors, where the energy generated is very limited.

I know of some 'power shaping' control rods, but they typically have a 'grey' absorber, e.g., nickel, rather than a black absorber, and they would not introduce positive reactivity in the core, but rather redistribute the negative reactivity.
 
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@Astronuc, you are correct in the very core of the matter it wasn't an accident, the only accident was the fact that Dyatlov did not know that the reactor is so unstable under low loads otherwise I believe he would have spared his own life , he wasn't suicidal after all, just quick tempered.
Sadly I must say here the mentality is different, Russians in particular have never shown the level of concern towards safety as their western counterparts.

More than 10 years before Chernobyl in 1975, in the Leningrad Nuclear power plant which also had the RBMK-1000 the same problem occurred, only this time differently, during reactor power rise after a maintenance shutdown one of the pressure channels burst causing steam and water flooding the reactor core, somehow they only noticed this after 20 mins of operating under power, hard to say what caused the accident as there is very little information about it as it was never made public.

Practically the same thing happened in Chernobyl unit-1 in 1982, where a fuel channel burst during reactor power up to design power.
I read conflicting bits about the reasons behind this, some suggest that in one of the cases some personell closed the valve to the specific channel by accident starving it of water yet no-one was charged with any wrongdoing,
could these events have been of the same nature as the 1986 accident just localized, aka local transients in the core due to the positive reactivity and moderator rod design ?
We can only guess as any information about these events is lost to archives and history most likely.
 

Astronuc

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After my previous post, I found some additional information on some improvements to the control rod design. See Figure 1 (page 3) in the following: https://inis.iaea.org/collection/NCLCollectionStore/_Public/25/028/25028966.pdf

And on top of page 5, "For "fast-acting rods," a new drive was introduced with the reduced gear ratio. The braking action starts at the end of rod movement. Because of this change and the gas-type water pusher, the control rod insertion time decreased to between 2 to 5 sec." These fast acting control rods, which I take to be emergency shutdown control rods, are more inline with Western standards.
 

Astronuc

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Sadly I must say here the mentality is different, Russians in particular have never shown the level of concern towards safety as their western counterparts.
I think that has changed, at least with the Russians I've known and interacted with. CCCP (and Russia) eventually developed VVERs, notably VVER-1000 and VVER-1200, which are more along the lines of Western PWRs. https://en.wikipedia.org/wiki/VVER

I'm aware that there was a lot of pressure from the US and EU to shutdown and decommission the VVER-440s after the disintegration of Soviet Union.
 
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Well the attitude is changing but rather slowly, I wonder what kept them from implementing such control rod features from the very beginning


In all honesty isn't the VVER essentially a PWR, surely I know it's a PWR type design but I mean isn't it very similar to the US PWR model that has been in operation for decades now ?
The Soviet developed the VVER in the 1960's I think , soon after the US had started building and operating PWR's, I don't want to speculate but given Soviet excellence in spying I have a strange feeling, well you know what I'm talking about.

Although I guess that just as with planes there isn't many different ways how to build a good PWR type reactor so they should essentially be similar.


As for the shutdown , I think only Germany and maybe one or two other countries decomissioned the early VVER's, Russia and some former Soviet republics still use them, also Finland has two of the early reactors from 1970's in their Loviisa NPP, last time I checked they were modernized and in operation.
 

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